90db 4 ohm with tubes?

Phooey I say, If the amp has a substrantaial power transformer (it does) and good output transformers (it does) then you can play you music through those soeakers. You will have to decide if you like the sound however-you may or may not- but I say it won't be the amps fault. I have lower wattage tube amps driving speakers that like to dip south of 4 ohms which sound absolutely fabulous. I am certain the math would say no... but my ears say yes,

Dr. M, the concern is NOT the speaker's 4 ohm or whatever nominal impedance. The concern is with the wide VARIATION of that impedance as a function of frequency (from about 4 ohms to about 28 ohms according to the measurements of the particular speaker that were cited earlier, with the 28 ohm peak occurring in a critical part of the spectrum), and the INTERACTION of that VARIATION with the amplifier's high output impedance.

Yes, if the sonic results turn out to be poor "it won't be the amp's fault." It won't be the speaker's fault either. It will be the result of a mismatch.

Regards,
-- Al

Well, if it's worth anything, the amp is very powerful, and yes it does have 4ohm taps. Good comments, all. Interesting. Any other thoughts?

Dr. Mechans, I have beaten this subject to death with Al and Ralph (Atmasphere). Check my threads. Al is 120% correct.

The ability of a tube amp to drive a speaker with wide impedance fluctuations with tight output regulation is a function of the amp's output impedance rather than the muscle of the power or output trannies.

My amp is an ARC VS-115 which Soundstage bench tested to have an output impedance of 1.1 ohms off the 8 ohm tap and approximately 50 to 60 percent of that off the 4 ohm tap. As a result, Soundstage measured output regulations to be +/- 1 db off the 8 ohm tap. Possibly half that off the 4 ohm tap.

Coincidentally, when looking over the VS-115 stats off the ARC web site, output regulation was reported to be 1.2 db off the 8 ohm tap, obviously quite close to the Soundstage bench tests. The likely reason my amp's output impedance is so low is because ARC uses about 12 db of NF.

I think Al might concur with the surmise that if an amp's output impedance is higher, say 5.5 ohms, its output regulation would be much higher in dbs as a function of the speaker's impedance variations.

If you or others are interested in this topic, pull my threads and you'll see what I went through to get this far along. I remember many of the techies like Al and Ralph wrote some years back that the design of an amp necessitates trade-offs and compromises. Although NF has been pooh-poohed for various reasons, without using some NF, my output impedance would not be as low as it is, and correlatively my amp's DF would be much lower, and so forth and so on.

Cheers and thanks again Al.

Bruce

Thanks again Al.

I'd agree the problem here is a speaker with wide load impedance fluctuations coupled to a power amp with high output impedance. My stipulation of good transformer and power supply isn't the solution in this case.NFB is an option for lowering the amp's output impedance but IMO introduces more harm overall than good. Widely fluctuating speaker loads just seems like a bad design choice. Why not just design a speaker with a flatter impedance curve? This would allow more amplifier choice and flexibility.
Regards,